docs: def get_sum_w2(self, ix, iy=0, iz=0):
          """
          Obtain the true number of entries in the bin weighted by w^2
          """
          if self.GetSumw2N() == 0:
              raise RuntimeError(
                  "Attempting to access Sumw2 in histogram "
                  "where weights were not stored")
          xl = self.nbins(axis=0, overflow=True)
          yl = self.nbins(axis=1, overflow=True)
          idx = xl * yl * iz + xl * iy + ix
          if not 0 <= idx < self.GetSumw2N():
              raise IndexError("bin index out of range")
          return self.GetSumw2().At(idx)